The invention relates to densifying porous substrates by chemical vapour infiltration (CVI).
The field of application of the invention is that of making parts out of thermostructural composite material, i.e. out of a composite material that not only possesses mechanical properties that enable it to be used for making structural parts, but that also has the ability to conserve these properties at high temperatures. Typical examples of thermostructural composite materials are carbon/carbon (C/C) composites having a reinforcing fabric of carbon fibers densified by a pyrolytic carbon matrix, and ceramic matrix composites (CMCs) having a reinforcing fabric of refractory fibers (carbon or ceramic) densified by a ceramic matrix.
A well known process for densifying porous substrates, to make C/C composite or CNC parts is chemical vapour infiltration. The substrates to be densified are placed in a loading zone of an oven in which they are heated. A reactive gas containing one or more gaseous precursors of the material that is to constitute the matrix is introduce into the oven. The temperature and pressure inside the oven are adjusted to enable the reactive gas to diffuse within the pores of the substrates and deposit matrix-constituting material therein by one or more components of the reactive gas decomposing or reacting together, said components constituting the matrix precursor. The process is performed under low pressure in order to encourage the reactive gas to diffuse into the substrates. The transformation temperature of the precursor(s) to form the matrix material, such as pyrolytic carbon or ceramic, is usually greater than 900° C., and is typically close to 1000° C.
In order to enable substrates throughout the loading zone of the oven to be densified as uniformly as possible, whether in terms of increasing density or in terms of the microstructure of the matrix material that is formed, it is necessary for the temperature throughout the loading zone to be substantially uniform.
Such ovens usually also include a zone situated between the reactive gas inlet into the oven and the loading zone of the oven in which the reactive gas is heated. Typically the gas heating zone comprises a plurality of perforated plates through which the reactive gas passes.
The gas-heating plates, like the substrates, are heated because they are present in the oven. The oven is generally heated by means of a susceptor, e.g. made of graphite, which defines the side wall of the oven and which is inductively coupled to an induction coil surrounding the oven, or is heated by resistors surrounding the susceptor.
The Applicants have found that the presence of a zone for heating the reactive gas does not always give the desired result. A significant example is that of densifying substrates constituted by annular preforms of carbon fibers or pre-densified annular blanks for use in making C/C composite brake disks. The substrates are placed in one or more vertical stacks in the loading zone above the gas heating zone which is situated at the bottom of the oven. In spite of the reactive gas being heated, a temperature gradient is observed between the bottom of the loading zone and the remainder thereof, with the temperature close to substrates situated at the bottom of the stack possibly being several tens of ° C. lower than the temperature that applies in the remainder of the stack. This gives rise to a large densification gradient between the substrates, depending on the position of a substrate within the stack.
In order to solve that problem, it would be possible to increase the efficiency with which the reactive gas is heated by increasing the heating zone. Nevertheless, for given total volume of the oven, that would reduce the working volume available in the loading zone. Unfortunately, chemical vapour infiltration processes require large amounts of industrial investment and they are very lengthy to perform. It is therefore highly desirable for ovens to have high productivity, whether they be ovens already in service or new ovens yet to be built, and thus as high as possible a ratio of working volume dedicated to the load of substrates over the volume which is dedicated to heating the reactive gas.